Decide whether a graph is a tree.Source:
is_tree() decides whether a graph is a tree, and optionally returns a
possible root vertex if the graph is a tree.
is_tree(graph, mode = c("out", "in", "all", "total"), details = FALSE)
An igraph graph object
Whether to consider edge directions in a directed graph. ‘all’ ignores edge directions; ‘out’ requires edges to be oriented outwards from the root, ‘in’ requires edges to be oriented towards the root.
Whether to return only whether the graph is a tree (
FALSE) or also a possible root (
FALSE, a logical value that indicates
whether the graph is a tree. When
TRUE, a named
list with two entries:
Logical value that indicates whether the graph is a tree.
The root vertex of the tree; undefined if the graph is not a tree.
An undirected graph is a tree if it is connected and has no cycles. In the directed case, a possible additional requirement is that all edges are oriented away from a root (out-tree or arborescence) or all edges are oriented towards a root (in-tree or anti-arborescence). This test can be controlled using the mode parameter.
By convention, the null graph (i.e. the graph with no vertices) is considered not to be a tree.
g <- make_tree(7, 2) is_tree(g) #>  TRUE is_tree(g, details = TRUE) #> $res #>  TRUE #> #> $root #> + 1/7 vertex, from aaa0e90: #>  1 #>